Fuel cells are electrochemical devices in which the energy from a chemical reaction is converted to direct current electricity. During operation of a fuel cell, a continuous flow of fuel, e.g., hydrogen (or a liquid fuel such as methanol), is fed to the anode while, simultaneously, a continuous flow of an oxidant, e.g., air, is fed to the cathode. The fuel is oxidized at the anode causing a release of electrons through the agency of a catalyst. These electrons are then conducted through an external load to the cathode, where the oxidant is reduced and the electrons are consumed, again through the agency of a catalyst. The constant flow of electrons from the anode to the cathode constitutes an electrical current which can be made to do useful work.
The Polymer Electrolyte Membrane fuel cell (PEMFC) is the most likely type of fuel cell to find wide application as a more efficient and lower emission power generation technology in a range of markets including stationary and portable power devices and as an alternative to the internal combustion engine in transportation. PEM fuel cells use a solid polymer as an electrolyte and porous carbon electrodes containing a platinum catalyst. They need only hydrogen, oxygen from the air, and water to operate and do not require corrosive fluids like some fuel cells. They are typically fueled with pure hydrogen supplied from storage tanks or onboard reformers.
The Direct Methanol Fuel Cell (DMFC) is similar to the PEMFC in that the electrolyte is a polymer and the charge carrier is the hydrogen ion (proton). However, liquid methanol (CH3OH) is oxidized in the presence of water at the anode generating CO2, hydrogen ions and the electrons that travel through the external circuit as the electric output of the fuel cell. The hydrogen ions travel through the electrolyte and react with oxygen from the air and the electrons from the external circuit to form water at the anode completing the circuit.
In the PEMFC the combined laminate structure formed from the membrane and the two electrodes is known as a membrane electrode assembly (MEA). The MEA will typically comprise several layers, but can in general be considered, at its basic level, to have five layers, which are defined principally by their function. On either side of the membrane an anode and cathode electrocatalyst is incorporated to increase the rates of the desired electrode reactions. In contact with the electrocatalyst containing layers, on the opposite face to that in contact with the membrane, are the anode and cathode gas diffusion substrate layers.
The anode gas diffusion substrate is designed to be porous and to allow the reactant hydrogen or methanol to enter from the face of the substrate exposed to the reactant fuel supply, and then to diffuse through the thickness of the substrate to the layer which contains the electrocatalyst, usually platinum-ruthenium metal based, to maximize the electrochemical oxidation of hydrogen or methanol. The anode electrocatalyst layer is also designed to comprise some level of the proton-conducting electrolyte in contact with the same electrocatalyst reaction sites. With acidic electrolyte types protons are produced as the product of the reaction occurring at the anode and these can then be efficiently transported from the anode reaction sites through the electrolyte to the cathode layers.
The cathode gas diffusion substrate is also designed to be porous and to allow oxygen or air to enter the substrate and diffuse through to the electrocatalyst layer reaction sites. The cathode electrocatalyst combines the protons with oxygen to produce water and is also designed to comprise some level of the proton-conducting electrolyte in contact with the same electrocatalyst reaction sites. Product water then has to diffuse out of the cathode structure. The structure of the cathode has to be designed such that it enables the efficient removal of the product water.
The complete MBA can be constructed by several methods. The electrocatalyst layers can be bonded to one surface of the gas diffusion substrates to form what is known as a gas diffusion electrode. The MEA is then formed by combining two gas diffusion electrodes with the solid proton-conducting membrane. Alternatively, the MEA may be formed from two porous gas diffusion substrates between which is sandwiched a solid proton-conducting polymer membrane having electrocatalyst layers on both sides (also referred to as a catalyst coated membrane or CCM); or indeed the MBA may be formed from one gas diffusion electrode, one liquid diffusion substrate and a solid proton-conducting polymer having an electrocatalyst layer on the side facing the gas/liquid diffusion substrate.
Although the theory behind fuel cell operation has been known for many years, there has been difficulty producing commercially viable fuel cells due to technological barriers, and also due to the availability of more cost-effective energy sources such as petroleum. However, devices using petroleum products, such as the automobile, produce significant pollution and may eventually become obsolete with the depletion of petroleum resources. As a result, there is a need for an alternative means for producing energy. Fuel cells are a promising alternative source of energy in that they are relatively pollution-free and utilize hydrogen, a seemingly infinite fuel source.
Among the critical issues that must be addressed for the successful commercialization of fuel cells are cell cost cell performance and operating lifetime. For stationary applications, improved power density is also critical. For automotive applications, high voltage efficiencies are necessary. In terms of cell cost and performance, one of the major issues is the construction and fabrication of the electrocatalyst layers used as the electrodes of the fuel cell, not least because of the fact that most fuel cells currently employ expensive noble metals, particularly platinum, as the or one electrocatalyst material.
Many different methods have been proposed for producing fuel cell electrodes, but most suffer from drawbacks that limit or prevent their commercial application. For example, one known method involves impregnating a porous support with a solution of a salt of an electrocatalyst metal, forming an oxide or hydroxide from the metal salt and reducing the oxide or hydroxide to the metal. Such a method is disclosed in, for example, U.S. Pat. No. 4,052,336 but generally results in poor control over the composition and microstructure of the catalyst powder, which are characteristics that have a critical impact on the performance of the catalyst.
An alternative method involves forming particles of the electrocatalyst metal, either alone or as coating on a particulate support phase, dispersing the metal particles into an ink and then applying the ink onto an electrode support or polymer membrane by, for example, ink-jet printing. One such method is disclosed in U.S. Patent Application Publication No. 2003/0130114 and comprises the steps of a) forming a liquid precursor comprising a particulate carbon precursor and at least a first precursor to an active species phase; b) generating an aerosol of droplets from said liquid precursor; and c) heating the aerosol droplets in a spray dryer at a conversion temperature of not greater than about 400° C. to form electrocatalyst particles wherein said first precursor is converted to an active species phase dispersed on the carbon support phase. The resultant electrocatalyst particles have a well-controlled microstructure and morphology and can be dispersed in an aqueous or organic liquid vehicle to produce an ink-jettable ink.
However, although inks are attractive deposition media for electrocatalyst metal particles, they suffer from the problem that, in view of the density of the electrocatalyst particles (frequently up to 20 gm/cc), it is difficult to maintain the particles adequately dispersed and suspended in the liquid vehicle. This problem is particularly pronounced when the ink is required to contain significant quantities of other components of the final catalyst layer, such as a proton-conducting polymer. At the same time, the viscosity of the liquid vehicle is limited by the requirement that the ink must be capable of being used with inkjet or similar print device. Thus, ensuring that the ink has a long shelf life, without frequent redispersion of the metal particles, can pose a significant challenge.
U.S. Patent Application Publication No. 2004/0038808, published Feb. 26, 2004, describes an ink composition useful for the formation of a catalyst layer and comprising: a) a liquid vehicle; b) a molecular precursor to an active species phase, wherein said molecular precursor can be converted to said active species phase at a temperature of not greater than about 200° C.; and c) particulate carbon. According to paragraph [0036], a variety of dispersants/additives can be added to the ink to assist in achieving a stable dispersion. In particular, fourteen commercially available dispersants were studied and, of these, DARVAN 7, a sodium polymethacrylate, water-soluble dispersant and DARVAN 821A, an ammonium polyacrylate, water soluble dispersant were found to be preferred in that neither were found to interfere adversely with the electrochemical process in a DMFC membrane electrode assembly.
Recent testing has, however shown that electrocatalyst ink compositions employing polyacrylate dispersants, such as DARVAN 821A, tend to undergo rapid settling, especially when the ink contains more than 5 wt % of a proton-conducting polymer. According to the present invention, it has now been found that electrocatalyst ink compositions, and especially aqueous electrocatalyst ink compositions, possessing enhanced stability can be produced by the inclusion in the composition of a copolymer dispersant including at least one polyalkylene oxide segment, particularly a comb-branched copolymer dispersant comprising at least one acrylic polymer segment and at least one polyalkylene oxide segment. The resultant inks have low settling rates even when containing more than 5 wt % of a proton-conducting polymer.
Comb-branched copolymers have already been described for use in the preparation of pigment dispersions. For example, U.S. Pat. No. 6,582,510 describes pigment dispersions comprising a pigment, a carrier and an acrylic/polyether comb-branched copolymer dispersant, wherein the polyether portion of the copolymer is free of any acidic groups. In addition, U.S. Patent Application Publication No. 2005/0256225, published Nov. 17, 2005, describes an aqueous inkjet ink composition comprising a) an aqueous vehicle, b) a modified pigment comprising a pigment having attached at least one organic group, and c) at least one comb-branched copolymer dispersant comprising at least one acrylic polymer segment and at least one polyalkylene oxide segment.
It is, however, to be appreciated that, with conventional pigment ink compositions, the pigments typically have a density of 2 gm/cc or less and are normally present in amounts less than 20 wt % of the overall ink composition. In contrast, with electrocatalyst ink compositions, the electrocatalyst metal particles frequently have a density up to 20 gm/cc and may be present in amounts in excess of 60 wt % of the overall ink composition. It is therefore unexpected that dispersants, such as comb-branched copolymer dispersants, that are effective in stabilizing pigment ink compositions should also be effective in stabilizing electrocatalyst ink compositions.